The invention generally relates to an integrated fuel cell system.
A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a polymer electrolyte membrane (PEM), often called a proton exchange membrane, that permits only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is reacted to produce protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the protons to form water. The anodic and cathodic reactions are described by the following equations (1) and (2):
H2xe2x86x922H++2exe2x88x92 at the anode of the cell, andxe2x80x83xe2x80x83(1) 
O2+4H++4exe2x88x92xe2x86x922H2O at the cathode of the cellxe2x80x83xe2x80x83(2) 
A typical fuel cell has a terminal voltage of up to approximately one volt DC. For purposes of producing much larger voltages, several fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide more power.
The fuel cell stack may include flow plates (graphite composite or metal plates, as examples) that are stacked one on top of the other. The plates may include various surface flow channels and orifices to, as examples, route the reactants and products through the fuel cell stack. Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells. Electrically conductive gas diffusion layers (GDLs) may be located on each side of each PEM to act as a gas diffusion media and in some cases to provide a support for the fuel cell catalysts. In this manner, reactant gases from each side of the PEM may pass along the flow channels and diffuse through the GDLs to reach the PEM. The PEM and its adjacent pair are often assembled together in an arrangement called a membrane electrode assembly (MEA).
A fuel cell system may include a fuel processor that converts a hydrocarbon (natural gas or propane, as examples) into a fuel flow for the fuel cell stack. Exemplary fuel processor systems are described in U.S. Pat. Nos. 6,207,122, 6,190,623, 6,132,689, which are hereby incorporated by reference.
The two reactions which are generally used to convert a hydrocarbon fuel into hydrogen are shown in equations (3) and (4).
xc2xdO2+CH4xe2x86x922H2+COxe2x80x83xe2x80x83(3) 
H2O+CH4xe2x86x923H2+COxe2x80x83xe2x80x83(4) 
The reaction shown in equation (3) is sometimes referred to as catalytic partial oxidation (CPO). The reaction shown in equation (4) is generally referred to as steam reforming. Both reactions may be conducted at a temperature from about 600-1,100xc2x0 C. in the presence of a catalyst such as platinum. A fuel processor may use either of these reactions separately or both in combination. While the CPO reaction is exothermic, the steam reforming reaction is endothermic. Reactors utilizing both reactions to maintain a relative heat balance are sometimes referred to as autothermal (ATR) reactors. It should be noted that fuel processors are sometimes generically referred to as reformers, and the fuel processor output gas is sometimes generically referred to as reformate, without respect to which reaction is employed.
As evident from equations (3) and (4), both reactions produce carbon monoxide (CO). Such CO is generally present in amounts greater than 10,000 parts per million (ppm). Because of the high temperature at which the fuel processor is operated, this CO generally does not affect the catalysts in the fuel processor. However, if this reformate is passed to a prior art fuel cell system operating at a lower temperature (e.g., less than 100xc2x0 C.), the CO may poison the catalysts in the fuel cell by binding to catalyst sites, inhibiting the hydrogen in the cell from reacting. In such systems it is typically necessary to reduce CO levels to less than 100 ppm to avoid damaging the fuel cell catalyst. For this reason the fuel processor may employ additional reactions and processes to reduce the CO that is produced. For example, two additional reactions that may be used to accomplish this objective are shown in equations (5) and (6). The reaction shown in equation (5) is generally referred to as the shift reaction, and the reaction shown in equation (6) is generally referred to as preferential oxidation (PROX).
CO+H2Oxe2x86x92H2+CO2xe2x80x83xe2x80x83(5) 
CO+xc2xdO2xe2x86x92CO2xe2x80x83xe2x80x83(6) 
Various catalysts and operating conditions are known for accomplishing the shift reaction. For example, the reaction may be conducted at a temperature from about 300-600xc2x0 C. in the presence of supported platinum. Other catalysts and operating conditions are also known. Such systems operating in this temperature range are typically referred to as high temperature shift (HTS) systems. The shift reaction may also be conducted at lower temperatures such as 100-300xc2x0 C. in the presence of other catalysts such as copper supported on transition metal oxides. Such systems operating in this temperature range are typically referred to as low temperature shift (LTS) systems. Other catalysts and operating conditions are also known. In a practical sense, typically the shift reaction may be used to lower CO levels to about 1,000-10,000 ppm, although as an equilibrium reaction it may be theoretically possible to drive CO levels even lower.
The PROX reaction may also be used to further reduce CO. The PROX reaction is generally conducted at lower temperatures than the shift reaction, such as 100-200xc2x0 C. Like the CPO reaction, the PROX reaction can also be conducted in the presence of an oxidation catalyst such as platinum. The PROX reaction can typically achieve CO levels less than 100 ppm (e.g., less than 50 ppm).
In general, fuel cell power output is increased by raising fuel and air flow to the fuel cell in proportion to the stoichiometric ratios dictated by the equations listed above. Thus, a controller of the fuel cell system may monitor the output power of the stack and based on the monitored output power, estimate the fuel and air flows required to satisfy the power demand. In this manner, the controller regulates the fuel processor to produce this flow, and in response to the controller detecting a change in the output power, the controller estimates a new rate of fuel flow and controls the fuel processor accordingly.
The ratio of fuel or air provided to a fuel cell over what is theoretically required by a given power demand is sometimes referred to as xe2x80x9cstoichxe2x80x9d. For example, 1 anode stoich refers to 100% of the hydrogen theoretically required to meet a given power demand, whereas 1.2 stoich refers to 20% excess hydrogen over what is theoretically required. Since in real conditions it is typical that not all of the hydrogen or air supplied to a fuel cell will actually react, it may be desirable to supply excess fuel and air to meet a give power demand.
The fuel cell system may provide power to a load, such as a load that is formed from residential appliances and electrical devices that may be selectively turned on and off to vary the power that is demanded. Thus, in some applications the load may not be constant, but rather the power that is consumed by the load may vary over time and change abruptly. For example, if the fuel cell system provides power to a house, different appliances/electrical devices of the house may be turned on and off at different times to cause the load to vary in a stepwise fashion over time.
There is a continuing need for integrated fuel cell systems designed to achieve objectives including the forgoing in a robust, cost-effective manner.
The present invention provides in one aspect an integrated fuel cell system having a first reactor adapted to receive a hydrocarbon feed and at least partially convert the hydrocarbon feed into a first fuel stream comprising a first hydrogen concentration and a first carbon monoxide concentration. A second reactor is adapted to receive the first fuel stream from the first reactor and react a portion of the first carbon monoxide concentration with steam to produce a second fuel stream having a second hydrogen concentration and a second carbon monoxide concentration. The second hydrogen concentration is greater than the first hydrogen concentration, the second carbon monoxide concentration is lower than the first carbon monoxide concentration, and the second carbon monoxide concentration is at least 1,000 part per million. A fuel cell has a temperature of at least 100xc2x0 C., wherein the fuel cell is adapted to receive the second fuel stream and react a portion of the second hydrogen concentration. The exhaust from the fuel cell comprises at least 1,000 parts per million carbon monoxide. An oxidizer is adapted to receive the exhaust and oxidize a portion of the carbon monoxide in the exhaust (e.g., preferably all of the CO).
Various embodiments of the invention can include the following features, alone or in combination. The first reactor being at least one of: a steam reforming reactor, a catalytic partial oxidation reactor, and an autothermal reactor. The hydrocarbon feed can comprise oxygen and steam. The hydrocarbon feed can comprise natural gas. The hydrocarbon feed can comprise a ratio of oxygen molecules to methane molecules in the range 0.50-0.6. The hydrocarbon feed can comprise a ratio of water vapor molecules to methane molecules in the range 2.5-4.0.
The second reactor can comprise a multi-stage shift reactor (see equation 5). The first reactor can comprise a conversion catalyst consisting essentially of platinum. In such embodiments, fuel cell catalyst components such as ruthenium alloys or mixtures are not needed due to the fact that the operating temperature of the fuel cell is relatively high (e.g., above 100xc2x0 C.).
In addition to the first and second reactors, the fuel cell system can further include a third reactor adapted to receive the second fuel stream and react a portion of the second carbon monoxide concentration with oxygen to produce a third fuel stream having a lower carbon monoxide concentration than the second carbon monoxide concentration. In such embodiments, the third fuel stream is then injected into the fuel cell.
The fuel cell operating temperature can also be in the range 100-200xc2x0 C., or preferably in the range 160-180xc2x0 C. The fuel cell can include a polybenzimidazole polymer exchange membrane or other PEMs suitable for operating in such temperature ranges. The second carbon monoxide concentration can be over 3000 parts per million in some embodiments, and over 8000 parts per million in other embodiments.
In another aspect, the invention provides a method of operating a fuel cell system, containing the following steps: flowing a hydrocarbon through a conversion reactor to produce a first fuel stream comprising hydrogen and carbon monoxide; flowing the first fuel stream through a shift reactor to react a portion of the carbon monoxide with steam to produce a second fuel stream still having at least 1,000 parts per million carbon monoxide; flowing the second fuel stream directly from the shift reactor through a conduit to a fuel cell operating at a temperature greater than 100xc2x0 C. to produce an exhaust stream comprising at least 1,000 parts per million carbon monoxide; and flowing the exhaust through an oxidizer to reduce the carbon monoxide to less than 100 parts per million.
In addition, such embodiments may also contain methods of operating a fuel cell system, embodying any of the following steps, either alone or in combination: flowing oxygen and steam through the conversion reactor, wherein the reactor is an autothermal reactor, wherein the hydrocarbon comprises natural gas, and wherein the natural gas comprises methane; flowing the methane through the conversion reactor at a first molar flow rate; flowing the oxygen through the conversion reactor at a second molar flow rate having a ratio in the range of 0.5-0.6 with respect to the first molar flow rate; and flowing the steam through the conversion reactor at a third molar flow rate having a ratio in the range of 2.5-4.0 with respect to the first molar flow rate. Such methods may relate to systems incorporating any of the features and operating conditions described above, either alone or in combination.
In another aspect, the invention provides another method of operating a fuel cell system, containing the following steps: converting a flow of hydrocarbon into a fuel stream comprising hydrogen and carbon monoxide; reacting a portion of the carbon monoxide with steam to produce additional hydrogen in the fuel stream; flowing the fuel stream through a polymer membrane fuel cell having a temperature greater than 100xc2x0 C.; exhausting the fuel stream from the fuel cell, wherein the exhausted fuel stream comprises at least 1,000 parts per million carbon monoxide; and flowing the exhausted fuel stream to an oxidizer wherein the carbon monoxide in the exhausted fuel stream is reduced to a level less than 100 parts per million. Such embodiments may also incorporate any of the features described above, either alone or in combination.
Advantages and other features of the invention will become apparent from the following description, drawing and claims.